Since the 16 domains are configured as the client domains in
mmu_setttbase(), a Permission fault is generated if the XN bit
(Execute-never) is set in the short-descriptor translation table
(for section and supersection). This leads to the booting failure
when enabling MMU for beagleboard bsp. Here is log:

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SD/MMC found on device 0
reading uEnv.txt
117 bytes read in 3 ms (38.1 KiB/s)
Loaded environment from uEnv.txt
Importing environment from mmc ...
Running uenvcmd ...
reading rtthread.bin
162624 bytes read in 24 ms (6.5 MiB/s)
\## Starting application at 0x80200000 ...

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This commit removes the XN bit configuration in the section of the
short-descriptor translation table. The OS can be booted successfully
with applying this commit.